(1) Field of the Invention
The present invention relates to an image signal processing apparatus and an image display device, and relates particularly to an image signal processing apparatus and an image display device for displaying images using a subfield drive.
(2) Description of the Related Art
In the image display device 100 displaying an image using a subfield drive as in plasma displays, light emission of each pixel of the image display device 100 is controlled in a manner as shown in FIG. 23A. Specifically, a frame (field), which is a unit display period of each image, is divided into subfields each having a different light emission period. Gradation control of the frame is performed by controlling light emission and non-light emission of the subfields which are used in different combinations.
More specifically, whether or not light is emitted is determined for each of the subfields depending on a pixel value of the pixel. In this method, light is emitted for a longer total time of subfields to provide a brighter frame (field), and for a shorter total time of subfields to provide a darker frame (field). Light emission and non-light emission of the subfields may be controlled by, for example, the image signal processing IC (image signal processing unit) 307 or the display panel (display unit) 308 shown in FIG. 3.
In addition, each subfield is weighted according to its brightness (that is, light emission period). Subfields 1 to 5 (SF1 to SF5) have different weights (different brightness levels). In this case, the SF1, which has the longest light emission period, is the brightest subfield in perceptual terms, and the closer to the SF5 having the shortest light emission period, the darker the subfield is in perceptual terms.
The SF1 to SF5 are arranged in a predetermined sequence. For example, in the case where light is emitted in order from the SF1 to the SF5, the subfields are arranged in descending order. The lower the place of the subfield is (the right-hand of the graph shown in FIG. 23A), the darker the subfield is.
On the other hand, in the example shown in FIG. 23B, a subfield in a lower place is brighter. In this example, the subfields are arranged in ascending order. Furthermore, an image in which gradation is achieved by causing each pixel to emit light during one of the subfields having different weights (brightness levels) or plural ones in combination. For example, in the case shown in FIG. 23A, light is emitted from a pixel during none of the SF1 to the SF5 in the darkest field. The pixel is caused to emit light during the SF5 in the second darkest, the SF4 in the third darkest field, and the SF5 and SF4 in the fourth darkest field. Different levels of brightness are thus achieved.
That is, the brighter the image is, the greater the total weight of the subfields during which light is emitted is. The darker the image is, the smaller the total weight of the subfields during which light is emitted is. In order to display an image having the highest level of brightness, light is emitted during all the subfields. In order to display an image having the lowest level of brightness, light is emitted during none of the subfields.
In the case where a stereoscopic image is displayed on a plasma display by alternately displaying left and the right images different from each other by parallax, it is preferable that subfields be arranged in descending order rather than in ascending order because of the effect of persistence between the left and right images. Because phosphors take a certain time to emit or decay light, the amount of persistence remaining in the next image frame is likely to be large in the case where light is emitted during a subfield having a greater weight in a lower place of the sequence.
On the other hand, there is a problem particular to a method of light emission of subfields. In the case where a plasma display displays an image for one field, four different operations are generally necessary for the subfields included in the field in order to cause phosphors to emit light. The four different operations are each performed in four different periods shown in FIG. 23A. The four periods are as follows: (A) an initialization period which is prior to all the subfields; (B) a write period and (C) a sustainment period each iterated for each of the subfields; and (D) an erase period in which phosphors are caused to decay light after the sustainment period of the last subfield.
In order to cause a pixel to emit light during each of the subfields, phosphors of the pixel are caused to emit light by the operations through the above periods. However, phosphors may fail to emit light for a reason, for example, that charge accumulated in the electrode of the pixel is insufficient or that accumulated charge has been decayed for any reason. In this case, the pixel fails to emit light during the subfield, and the image relatively darkens only in the field, affecting the quality of the image to be viewed. Particularly in the case where a method is employed in which a pixel is caused to emit light during subfields arranged in descending order, degradation in image quality may be larger when the pixel fails in light emission of the subfield at the top of the sequence (the brightest subfield). Therefore, such failure in light emission affects more severely in the case where the subfields are arranged in descending order than in ascending order.
When any of the above-described methods is used with an image display device which displays a stereoscopic image by subfield drive, there is a conflict between the measures against persistence in a stereoscopic image and the measures for improvement in reliability of subfield drive.